System and method for estimating rotor blade loads of a wind turbine

ABSTRACT

The present subject matter is directed to a method for estimating rotor blade loads, e.g. a blade root resultant moment, of a wind turbine. In one embodiment, the method includes measuring, via one or more sensors, a plurality of operating parameters of the wind turbine. Another step includes estimating out-of-plane and in-plane forces acting on the rotor blade based at least partially on the plurality of operating parameters. Further, the method includes determining an application point for the out-of-plane and in-plane forces along a span of the rotor blade. A further step includes estimating out-of-plane and in-plane moments of the rotor blade based at least partially on the out-of-plane and in-plane forces and the respective application points. Thus, the method includes calculating the load acting on the rotor blade based at least partially on the out-of-plane and in-plane moments.

FIELD OF THE INVENTION

The present subject matter relates generally to wind turbines and, moreparticularly, to a system and method for estimating rotor blade loadsacting on a wind turbine.

BACKGROUND OF THE INVENTION

Wind power is considered one of the cleanest, most environmentallyfriendly energy sources presently available and wind turbines havegained increased attention in this regard. A modern wind turbinetypically includes a tower, a generator, a gearbox, a nacelle, and oneor more rotor blades. The rotor blades are the primary elements forconverting wind energy into electrical energy. The blades typically havethe cross-sectional profile of an airfoil such that, during operation,air flows over the blade producing a pressure difference between itssides. Consequently, a lift force, which is directed from the pressureside towards the suction side, acts on the blade. The lift forcegenerates torque on the main rotor shaft, which is connected to agenerator for producing electricity.

The amount of power that may be produced by a wind turbine is typicallylimited by structural limitations (i.e. design loads) of the individualwind turbine components. For example, the blade root of a wind turbinemay experience loads (e.g. a blade root resultant moment) associatedwith both average loading due to turbine operation and dynamicallyfluctuating loads due to environmental conditions. Such loading maydamage turbine components, thereby eventually causing the turbinecomponents to fail. The fluctuating loads can change day-to-day orseason-to-season and may be based on wind speed, wind peaks, windturbulence, wind shear, changes in wind direction, density in the air,yaw misalignment, upflow, or similar. Specifically, for example, loadsexperienced by a wind turbine may vary with wind speed.

As such, it is imperative to ensure loads acting on the wind turbine donot exceed design loads. Thus, many wind turbines employ one or moresensors configured to measure the loads acting on the various windturbine components. Though the sensors may provide the desiredinformation, new sensor systems can be complex and expensive to install.Further, the sensors may provide inaccurate information and can be proneto fail.

Additionally, wind turbines utilize control systems configured toestimate loads acting on the wind turbine based on a wind turbinethrust. The terms “thrust,” “thrust value,” “thrust parameter” orsimilar as used herein are meant to encompass a force acting on the windturbine due to the wind. The thrust force comes from a change inpressure as the wind passes the wind turbine and slows down. Suchcontrol strategies estimate loads acting on the wind turbine bydetermining an estimated thrust using a plurality of turbine operatingconditions, such as, for example, pitch angle, power output, generatorspeed, and air density. The operating conditions are inputs for thealgorithm, which includes a series of equations, one or more aerodynamicperformance maps, and one or more look-up tables (LUTs). For example,the LUT may be representative of a wind turbine thrust. A +/−standarddeviation of the estimated thrust may also be calculated, along with anoperational maximum thrust and a thrust limit. As such, the wind turbinemay be controlled based on a difference between the maximum thrust andthe thrust limit.

In view of the foregoing, the art is continuously seeking new andimproved systems for estimating loads acting on a wind turbine. Thus, asystem and method for estimating rotor blade loads of a wind turbine,e.g. a blade root resultant moment of a rotor blade, would be desired inthe art. Further, a system and method that incorporated existinghardware and software would be advantageous.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one aspect, the present subject matter is directed to a method forestimating a rotor blade load, e.g. a blade root resultant moment,acting on a wind turbine. The method includes measuring, via one or moresensors, a plurality of operating parameters of the wind turbine.Another step includes estimating, via a processor, an out-of-plane andan in-plane force acting on the rotor blade based at least partially onthe plurality of operating parameters. Further, the method includesdetermining, via the processor, an application point of the out-of-planeand in-plane forces on a span of the rotor blade. A further stepincludes estimating, via the processor, out-of-plane and in-planemoments of the rotor blade based at least partially on the out-of-planeand in-plane forces and the out-of-plane and in-plane applicationpoints. Thus, the method includes calculating, via the processor, therotor blade load acting on the rotor blade based at least partially onthe out-of-plane moment and the in-plane moment.

In one embodiment, the plurality of operating parameters of the windturbine may include at least one of the following: a thrust, a power, aspeed, a torque, a pitch angle, a nodding moment, an overhang moment, arotor azimuth angle, a yawing moment, or any other operating parameterof the wind turbine. In another embodiment, the method may includeestimating the thrust by utilizing at least one of the following: aplurality of equations, one or more aerodynamic performance maps, or oneor more look-up tables.

In additional embodiments, the step of estimating the out-of-plane rootforce may include determining an out-of-plane force distribution from ablade root to a blade tip of the rotor blade and integrating theout-of-plane force distribution to obtain an equivalent out-of-planeforce.

In still another embodiment, the step of determining the applicationpoint of the out-of-plane force may include determining a plurality ofapplication points of a plurality of out-of-plane forces duringoperation of the wind turbine for a plurality of wind speeds and storingthe plurality of application points in an aerodynamic performance map.In an alternative embodiment, the application point of the out-of-planeforce may be a constant value.

In yet another embodiment, the method may include estimating thein-plane moment of the rotor blade as a function of at least one of aweight of the rotor blade, a mechanical torque load of the rotor blade,a speed of the rotor blade, a power of the rotor blade, or an inertialload of the rotor blade. More specifically, in one embodiment, themethod may include calculating the gravity load of the rotor blade as afunction of at least one of the rotor azimuth angle, a mass of the rotorblade, gravity, a hub radius, or a center of gravity location of therotor blade. Further, the method may include calculating the mechanicaltorque load of the rotor blade as a function of at least one of alow-speed shaft torque or power, a blade radius, or the hub radius. Inaddition, the method may include calculating the inertial load of therotor blade as a function of at least one of rotor acceleration or rotorinertia. In still further embodiments, the method may includedetermining the rotor acceleration based at least partially on a rate ofchange of a rotor speed signal and filtering the speed signal. Morespecifically, in one embodiment, the method may include filtering thespeed signal via a low pass filter.

In additional embodiments, the method may also include estimating anin-plane shear force of the rotor blade as a function of at least one ofthe rotor azimuth angle, a rotor blade mass, a rotor blade radius, theapplication point, torque, filtered rotor acceleration, rotor inertia,gravity, aerodynamic drag, wind speed, and/or a center of gravitylocation. Further, the method may include calculating the load acting onthe rotor blade based at least partially on the in-plane shear force.

In another aspect, the present disclosure is directed to a system formaintaining rotor blade loads of a wind turbine within predeterminedlimits while also maximizing power output. The system includes one ormore sensors configured to measure a plurality of operating parametersof the wind turbine and a controller configured with the one or moresensors. In addition, the controller includes one or more processorsconfigured to perform one or more operations. For example, in oneembodiment, the operations may include estimating an out-of-plane forceand an in-plane force acting on the rotor blade based at least partiallyon the plurality of operating parameters, determining an applicationpoint of the out-of-plane and in-plane forces on a span of the rotorblade, estimating an out-of-plane and an in-plane moment of the rotorblade based at least partially on the out-of-plane and in-plane forcesand the out-of-plane and in-plane application points, and calculatingthe rotor blade load acting on the rotor blade based at least partiallyon the out-of-plane moment and the in-plane moment. It should beunderstood that the system may be further configured with any of theadditional features as described herein and may implement any of theadditional method steps as described herein as well.

In yet another aspect, the present disclosure is directed to a methodfor estimating an out-of-plane blade root moment of the rotor blade. Themethod includes measuring a plurality of operating parameters of thewind turbine. For example, the operating parameters may include at leastone of the following: a thrust, a nodding moment, an overhang moment, arotor azimuth angle, or a yawing moment. Another step includesestimating an out-of-plane force acting on the rotor blade based atleast partially on the plurality of operating parameters. The methodalso includes determining an application point on the rotor blade of theout-of-plane force. Thus, the out-of-plane blade root moment isdetermined as a function of the out-of-plane force and the applicationpoint of the out-of-plane force.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 illustrates a perspective view of one embodiment of a windturbine according to the present disclosure;

FIG. 2 illustrates a simplified, internal view of one embodiment of anacelle of a wind turbine according to the present disclosure;

FIG. 3 illustrates a schematic diagram of one embodiment of a controlleraccording to the present disclosure;

FIG. 4 illustrates a graph one embodiment of the application points forthe out-of-plane and in-plane forces acting on a rotor blade as afunction of wind speed according to the present disclosure;

FIG. 5 illustrates a schematic diagram of one embodiment of anout-of-plane force distribution of a rotor blade according to thepresent disclosure;

FIG. 6 illustrates a free body diagram (FBD) of one embodiment of arotor of a wind turbine according to the present disclosure;

FIG. 7 illustrates a graph comparing the estimated blade root resultantmoment and the measured blade root resultant moment according to oneembodiment of the present disclosure;

FIG. 8 illustrates a graph of the measured blade root resultant momentplotted on the x-axis and the estimated blade root resultant momentplotted on the y-axis according to one embodiment of the presentdisclosure;

FIG. 9 illustrates a graph comparing the estimated thrust and themeasured blade root resultant moment according to conventionalconstruction;

FIG. 10 illustrates a graph of the measured blade root resultant momentplotted on the x-axis and the estimated thrust plotted on the y-axisaccording to conventional construction;

FIG. 11 illustrates a simplified flow diagram of one embodiment of stepstaken by the controller to calculated a blade root resultant momenterror according to the present disclosure; and

FIG. 12 illustrates a flow diagram of one embodiment of a method forestimating a rotor blade load of a wind turbine according to the presentdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

Generally, the present subject matter is directed to improved systemsand methods for estimating rotor blade loads of a wind turbine. In oneembodiment, for example, the system collects and stores operatingparameters of the wind turbine. Thus, the system is configured toestimate an out-of-plane and an in-plane force acting on the rotor bladebased on the operating parameters. The system is also configured todetermine an application point of the out-of-plane and in-plane forcesalong a span of the rotor blade. As such, the system can then estimatean out-of-plane and in-plane blade root moment based on the out-of-planeand in-plane forces and the out-of-plane and in-plane applicationpoints. Accordingly, the system is configured to determine a blade rootresultant moment of the rotor blade based at least partially on theout-of-plane blade root moment and the in-plane blade root moment.

The various embodiments of the system and method described hereinprovide numerous advantages. For example, the present disclosuremaintains rotor blade loads within predetermined limits, while alsomaximizing annual energy production (AEP). Thus, the present disclosurecan help prevent turbine component failure caused by excessive bladeloading. Moreover, the present disclosure may be implemented usingexisting components of the wind turbine and does not require additionalsensors. As such, a user is not required to purchase, install, andmaintain new equipment. Moreover, the system may be integrated with abroader control system, such as, but not limiting of, a wind turbinecontrol system, a plant control system, a remote monitoring system, orcombinations thereof.

Referring now to FIG. 1, a perspective view of one embodiment of a windturbine 10 that may implement the control technology according to thepresent disclosure is illustrated. As shown, the wind turbine 10generally includes a tower 12 extending from a support surface 14, anacelle 16 mounted on the tower 12, and a rotor 18 coupled to thenacelle 16. The rotor 18 includes a rotatable hub 20 and at least onerotor blade 22 coupled to and extending outwardly from the hub 20. Forexample, in the illustrated embodiment, the rotor 18 includes threerotor blades 22. However, in an alternative embodiment, the rotor 18 mayinclude more or less than three rotor blades 22. Each rotor blade 22 maybe spaced about the hub 20 to facilitate rotating the rotor 18 to enablekinetic energy to be transferred from the wind into usable mechanicalenergy, and subsequently, electrical energy. For instance, the hub 20may be rotatably coupled to an electric generator 24 (FIG. 2) positionedwithin the nacelle 16 to permit electrical energy to be produced.

The wind turbine 10 may also include a wind turbine controller 26centralized within the nacelle 16. However, in other embodiments, thecontroller 26 may be located within any other component of the windturbine 10 or at a location outside the wind turbine. Further, thecontroller 26 may be communicatively coupled to any number of thecomponents of the wind turbine 10 in order to control the operation ofsuch components and/or to implement a correction action. As such, thecontroller 26 may include a computer or other suitable processing unit.Thus, in several embodiments, the controller 26 may include suitablecomputer-readable instructions that, when implemented, configure thecontroller 26 to perform various different functions, such as receiving,transmitting and/or executing wind turbine control signals. Accordingly,the controller 26 may generally be configured to control the variousoperating modes (e.g., start-up or shut-down sequences), de-rate thewind turbine, and/or control various components of the wind turbine 10as will be discussed in more detail below.

Referring now to FIG. 2, a simplified, internal view of one embodimentof the nacelle 16 of the wind turbine 10 shown in FIG. 1 is illustrated.As shown, the generator 24 may be coupled to the rotor 18 for producingelectrical power from the rotational energy generated by the rotor 18.For example, as shown in the illustrated embodiment, the rotor 18 mayinclude a rotor shaft 34 coupled to the hub 20 for rotation therewith.The rotor shaft 34 may, in turn, be rotatably coupled to a generatorshaft 36 of the generator 24 through a gearbox 38. As is generallyunderstood, the rotor shaft 34 may provide a low speed, high torqueinput to the gearbox 38 in response to rotation of the rotor blades 22and the hub 20. The gearbox 38 may then be configured to convert the lowspeed, high torque input to a high speed, low torque output to drive thegenerator shaft 36 and, thus, the generator 24.

Each rotor blade 22 may also include a pitch adjustment mechanism 32configured to rotate each rotor blade 22 about its pitch axis 28.Further, each pitch adjustment mechanism 32 may include a pitch drivemotor 40 (e.g., any suitable electric, hydraulic, or pneumatic motor), apitch drive gearbox 42, and a pitch drive pinion 44. In suchembodiments, the pitch drive motor 40 may be coupled to the pitch drivegearbox 42 so that the pitch drive motor 40 imparts mechanical force tothe pitch drive gearbox 42. Similarly, the pitch drive gearbox 42 may becoupled to the pitch drive pinion 44 for rotation therewith. The pitchdrive pinion 44 may, in turn, be in rotational engagement with a pitchbearing 46 coupled between the hub 20 and a corresponding rotor blade 22such that rotation of the pitch drive pinion 44 causes rotation of thepitch bearing 46. Thus, in such embodiments, rotation of the pitch drivemotor 40 drives the pitch drive gearbox 42 and the pitch drive pinion44, thereby rotating the pitch bearing 46 and the rotor blade 22 aboutthe pitch axis 28. Similarly, the wind turbine 10 may include one ormore yaw drive mechanisms 66 communicatively coupled to the controller26, with each yaw drive mechanism(s) 66 being configured to change theangle of the nacelle 16 relative to the wind (e.g., by engaging a yawbearing 68 of the wind turbine 10).

Still referring to FIG. 2, the wind turbine 10 may also include one ormore sensors 48, 50 for measuring various operating parameters that maybe required to calculate the out-of-plane and in-plane moments asdescribed in more detail below. For example, in various embodiments, thesensors may include blade sensors 48 for measuring a pitch angle of oneof the rotor blades 22 or for measuring a load acting on one of therotor blades 22; generator sensors (not shown) for monitoring thegenerator 24 (e.g. torque, rotational speed, acceleration and/or thepower output); sensors for measuring the imbalance loading in the rotor(e.g. main shaft bending sensors); and/or various wind sensors 50 formeasuring various wind parameters, such as wind speed, wind peaks, windturbulence, wind shear, changes in wind direction, air density, orsimilar. Further, the sensors may be located near the ground of the windturbine, on the nacelle, or on a meteorological mast of the windturbine. It should also be understood that any other number or type ofsensors may be employed and at any location. For example, the sensorsmay be Micro Inertial Measurement Units (MIMUs), strain gauges,accelerometers, pressure sensors, angle of attack sensors, vibrationsensors, proximity sensors, Light Detecting and Ranging (LIDAR) sensors,camera systems, fiber optic systems, anemometers, wind vanes, SonicDetection and Ranging (SODAR) sensors, infra lasers, radiometers, pitottubes, rawinsondes, other optical sensors, and/or any other suitablesensors. It should be appreciated that, as used herein, the term“monitor” and variations thereof indicates that the various sensors maybe configured to provide a direct measurement of the parameters beingmonitored or an indirect measurement of such parameters. Thus, thesensors may, for example, be used to generate signals relating to theparameter being monitored, which can then be utilized by the controller26 to determine the actual parameter.

Referring now to FIG. 3, there is illustrated a block diagram of oneembodiment of various components of the controller 26 according to thepresent disclosure. As shown, the controller 26 may include one or moreprocessor(s) 58 and associated memory device(s) 60 configured to performa variety of computer-implemented functions (e.g., performing themethods, steps, calculations and the like and storing relevant data asdisclosed herein). Additionally, the controller 26 may also include acommunications module 62 to facilitate communications between thecontroller 26 and the various components of the wind turbine 10.Further, the communications module 62 may include a sensor interface 64(e.g., one or more analog-to-digital converters) to permit signalstransmitted from the sensors 48, 50 to be converted into signals thatcan be understood and processed by the processors 58. It should beappreciated that the sensors 48, 50 may be communicatively coupled tothe communications module 62 using any suitable means. For example, asshown in FIG. 3, the sensors 48, 50 are coupled to the sensor interface64 via a wired connection. However, in other embodiments, the sensors48, 50 may be coupled to the sensor interface 64 via a wirelessconnection, such as by using any suitable wireless communicationsprotocol known in the art.

As used herein, the term “processor” refers not only to integratedcircuits referred to in the art as being included in a computer, butalso refers to a controller, a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit, and other programmable circuits. Additionally, the memorydevice(s) 60 may generally comprise memory element(s) including, but notlimited to, computer readable medium (e.g., random access memory (RAM)),computer readable non-volatile medium (e.g., a flash memory), a floppydisk, a compact disc-read only memory (CD-ROM), a magneto-optical disk(MOD), a digital versatile disc (DVD) and/or other suitable memoryelements. Such memory device(s) 60 may generally be configured to storesuitable computer-readable instructions that, when implemented by theprocessor(s) 58, configure the controller 26 to perform variousfunctions including, but not limited to, determining one or more currentwind turbine parameters of the wind turbine 10 based on the plurality ofoperating data, determining a maximum wind turbine parameter,transmitting suitable control signals to implement control actions toreduce loads acting on the wind turbine, and various other suitablecomputer-implemented functions.

The processor 58 is configured to utilize the measured operatingparameters from the sensors 48, 50 to estimate rotor blade loads (e.g.blade root resultant moments and/or forces) of the wind turbine 10. Forexample, the sensors 48, 50 are configured to measure various windturbine and/or environmental conditions, so as to directly or indirectlyprovide information regarding one or more of the following parameters: arotor thrust, a mechanical torque, in-plane and out-of-plane applicationpoints of forces acting on the rotor blades 22, rotor imbalancemeasurements, a rotor azimuth angle, rotor speed, a gearbox ratio, anodding moment, an overhang moment, a yawing moment, gravity, a hubradius, a blade radius, a cone angle, a blade mass, a blade weight, acenter of gravity location for each of the rotor blades 22, or any otheroperating parameter of the wind turbine 10. More specifically, Table 1below illustrates one sample set of inputs that may be used by theprocessor 58 to estimate the load estimations as described herein.

TABLE 1 Description of Blade Root Resultant Moment Estimation InputsParameter Description ApplicationPointOP Fraction radius along bladewhere integrated aerodynamic thrust force is applied ApplicationPointIPFraction radius along blade where integrated aerodynamic in-plane forceis applied RotorAccTC 1st order low pass filter time constant for rotoracceleration signal BladeRadius Blade Radius BladeMass Blade Mass RotorInertia Spinning inertia Cone Angle Rotor cone angle CGloc Applicationpoint of center of gravity from rotor apex HubCO Hub Radius GearboxRatio Gearbox Ratio Overhang Moment Rotor + Hub overhang moment abouthub flange location Gravity Gravity

In most embodiments, the inputs of Table 1 are readily available andeasy to calculate and/or measure; however, the present disclosureprovides unique methods for calculating one or more of the parameters,e.g. ApplicationPointOP and ApplicationPointIP. More specifically, inone embodiment, the ApplicationPointOP is a function of tip speed ratio(TSR) and pitch angle and can be defined throughout the turbineoperation as shown in Equation (1) below:

$\begin{matrix}{{{ApplicationPoint}\mspace{14mu} O\; {P\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}} = \frac{{Nblades}*{OutofPlane}\mspace{14mu} {{Moment}\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}}{{{Thrust}\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}*{Blade}\mspace{14mu} {Radius}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where

-   Nblades is equal to the number of blades on the wind turbine.

Further, ApplicationPointIP can be defined in a similar fashion withEquation (2) below:

$\begin{matrix}{{{ApplicationPoint}\mspace{11mu} {{IP}\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}} = \frac{{Torque}\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}{{Nblades}*{In}\text{-}{Plane}\mspace{14mu} {Shear}\mspace{20mu} {{Force}\left( {{T\; S\; R},{{pitch}\mspace{14mu} {angle}}} \right)}*{Blade}\mspace{14mu} {Radius}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

Thus, the calculated values for ApplicationPointOP andApplicationPointIP may be stored in the memory device 60 in a table.Accordingly, in one embodiment, the values may be averaged and theaverage ApplicationPointOP and ApplicationPointIP can be used tocalculate the blade root resultant moment which is described in moredetail below.

Referring now to FIG. 4, a graph of ApplicationPointOP andApplicationPointIP values as a function of wind speed for nominalturbine operation is illustrated. More specifically, as shown, theaverage ApplicationPointOP is approximately 0.75 (i.e. located at 75%span of the rotor blade 22), whereas the average ApplicationPointIP isapproximately 0.5 (i.e. located at 50% span of the rotor blade 22),which are provided as mere examples of suitable ApplicationPointOP andApplicationPointIP values.

In an alternative embodiment, ApplicationPointOP and ApplicationPointIPmay be calculated during the aerodynamic performance map process. Theaerodynamic performance maps as described herein are dimensional ornon-dimensional tables that describe rotor loading and performance (e.g.power, thrust, torque, or bending moment, or similar) under givenconditions (e.g. density, wind speed, rotor speed, pitch angles, orsimilar). As such, the aerodynamic performance maps may include: a powercoefficient, a thrust coefficient, a torque coefficient, and/or partialderivatives with respect to pitch angle, rotor speed, and tip speedratio (TSR), and in this case, the application points of the forcesacting on the rotor blade. Alternatively, the aerodynamic performancemaps can be dimensional power, thrust, and/or torque values instead ofcoefficients. Thus, in a certain embodiment, the processor 58 isconfigured to determine the out-of-plane application point bydetermining a plurality of application points of a plurality ofout-of-plane forces during operation of the wind turbine for a pluralityof wind speeds and storing the plurality of application points in anaerodynamic performance map. This approach allows for more accurateblade root loading estimations during off-nominal operating periods, assuch application points of force vary as a function of wind speed/rotorspeed (tip-speed ratio) and pitch angle (off-nominal operation istypically when peak loads occur). In further embodiments, the processor58 is configured to prevent introduction of a structural stiffness orturbine geometry dependency into the aerodynamic performance maps whenintegrating ApplicationPointOP and ApplicationPointIP. For example, asshown in FIG. 5, a schematic diagram of one embodiment of anout-of-plane force distribution from the blade root to the blade tip ofthe rotor blade 22 is illustrated. By integrating the area under theout-of-plane force distribution curve, the integrated or equivalentout-of-place force and corresponding application point r along the rotorblade 22 can be determined.

Accordingly, the processor 58 is configured to utilize the operatingparameters, as well as ApplicationPointOP and ApplicationPointIP, toestimate various load components of the blade root resultant moment,including at least, an out-of-plane shear force, an in-plane shearforce, an axial force, an out-of-plane blade root moment, an in-planeblade root moment, and a blade torsion. It should be understood that theblade root resultant moment calculations as described herein aredirected to the wind turbine 10 having three rotor blades 22; however,such calculations are provided for example purposes only and are notmeant to be limiting. Thus, the calculations as described herein may beapplied to a wind turbine having any suitable number of blades. Inaddition, in various embodiments, it is assumed that theApplicationPointOP and ApplicationPointIP is the same for all threerotor blades and does not vary with individual blade TSR and/or pitchangle.

More specifically, in certain embodiments, the processor 58 isconfigured to calculate the out-of-plane root moment of each rotor blade22 as a function of one or more of the out-of-plane application point rof the force (i.e. ApplicationPointOP), the blade radius, the center ofgravity location, gravity, rotor imbalance load, and/or the rotorazimuth angle. For example, referring now to FIG. 6, a free body diagram(FBD) of the wind turbine rotor 18 is illustrated. As shown, the rotorazimuth angle θ is referenced from rotor blade 1 (i.e. B1), with θincreasing as the blades (i.e. B1, B2, and B3) rotate in a clockwisemanner. Thus, θ is between 0° and 360° with 0° being defined as B1 atthe 12 o'clock position. The blade forces, namely forces F₁, F₂, F₃, arethe out-of-plane forces going “into the page” due to aerodynamics (butneglecting centrifugal load from spinning mass with a coned rotor). Suchforces are assumed to be applied at respective application points r,which are assumed to be the same for all three rotor blades.

Thus, the three equations shown below (Equations 3-5) having threeunknowns (i.e. forces F₁, F₂, F₃) can be developed and solved usingvarious methods known in the art.

Estimated Thrust=F ₁ +F ₂ +F ₃   Equation (3)

Measured Nodding Moment−Static Overhang Moment=F ₁ r cos(θ)+F ₂ rcos(θ−120°)+F ₃ r cos(θ−240°)   Equation (4)

Measured Yawing Moment=F ₁ r sin(θ)+F ₂ r sin(θ−120°)+F ₃ r sin(θ−240°)  Equation (5)

The estimated thrust can be determined using a variety of techniques.For example, in one embodiment, the wind parameter estimator 56 may beconfigured to implement a control algorithm having a series of equationsto determine the estimated thrust. As such, the equations are solvedusing one or more operating parameters, one or more aerodynamicperformance maps, one or more LUTs, or any combination thereof. Asmentioned, the aerodynamic performance maps describe rotor loading andperformance (e.g. power, thrust, torque, or bending moment, or similar)under given conditions (e.g. density, wind speed, rotor speed, pitchangles, or similar). In addition, the LUTs may include: blade loads,tower loads, shaft loads, or any other wind turbine component load.

The measured nodding moment can also be determined using a variety oftechniques. Further, it should be understood that the measured noddingmoment as referred to herein generally refers to the aerodynamicallyinduced nodding moment of the rotor 18. For example, in one embodiment,the measured nodding moment is equal to the nodding moment whichprovides 0 kNm when the wind speed is 0 meters/second.

In the illustrated embodiment, the Overhang Moment of Equation (4) andthe Measured Yawing Moment of Equation (5) can be determined from theone or more sensors 48, 50. More specifically, in a particularembodiment, the sensors 48, 50 may be proximity probe measurementdevices.

Once the processor 58 solves Equations 3-5 for the three unknowns,namely forces F₁, F₂, F₃, the processor 58 is configured to determinethe out-of-plane bending moment for each of the rotor blades 22. Morespecifically, in one embodiment, the processor 58 calculates theout-of-plane blade root bending moment using Equation (6) below:

Outofplane Bending Moment=F*ApplicationPointOP*Blade Radius−HubCO  Equation (6)

where

-   F is equal to the corresponding force acting on the rotor blade,    e.g. F₁, F₂, or F₃.

As mentioned, the processor 58 is also configured to estimate anin-plane blade root moment of the rotor blade 22. More specifically, ina particular embodiment, the processor 58 is configured to estimate thein-plane blade root moment of the rotor blade 22 as a function of atleast one of a weight of the rotor blade 22, a mechanical torque load ofthe rotor blade, and/or an inertial load of the rotor blade 22. Forexample, the processor 58 may calculate the weight of the rotor blade 22as a function of at least one of the rotor azimuth angle, a rotor blademass, gravity, a hub radius, or a center of gravity location of therotor blade, as shown in Equation (7) below:

Blade Weight Load=−sin(θ)*BladeMass*Gravity*(CG_(loc)−Hub_(CO))  Equation (7)

In a further embodiment, the processor 58 is configured to calculate themechanical torque load of the rotor blade 22 as a function of at leastone of a low-speed shaft torque, a blade radius, or the hub radius, asshown in Equation (8) below:

$\begin{matrix}{{{Mechanical}\mspace{14mu} {Torque}\mspace{14mu} {Load}} = {\frac{Torque}{Nblades}*\frac{\left( {{BladeRadius} - {Hub}_{CO}} \right)}{BladeRadius}}} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

In still another embodiment, the processor 58 is configured to calculatethe inertial load of the rotor blade 22 as a function of at least one ofa rotor acceleration or a rotor inertia, as shown in Equation (9) below:

$\begin{matrix}{{{Inertial}\mspace{14mu} {Load}} = {{RotorAcceleration}_{filtered}*\frac{RotorInertia}{Nblades}}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

In various embodiments, each rotor blade 22 contributes equally to therotor acceleration. Thus, in certain embodiments, the aerodynamicin-plane loads are equal on all rotor blades 22 and only change as fastas the mechanical torque. The in-plane inertial loading from rotoracceleration is important to the overall in-plane load. Thus, to capturethis effect, the processor 58 may determine the rotor acceleration basedat least partially on a rate of change of a rotor speed signal andfiltering the speed signal. More specifically, in one embodiment, theprocessor 58 may filter the speed signal via a low pass filter. As such,the low-pass filter may pass low-frequency signals but attenuate (i.e.reduce the amplitude of) signals with frequencies higher than a cutofffrequency. The low-frequency signals may be then subtracted from the rawsignal such that only the high-frequency signals remain. In furtherembodiments, the low-pass filter may be used in conjunction with ahigh-pass filter. Further, any number of low-pass filters or high-passfilters may be used in accordance with the present disclosure.Alternatively the processor 58 may filter the speed signal via ahigh-pass filter. As such, the high pass filter may pass high-frequencysignals but attenuate signals with frequencies lower than a cutofffrequency.

After calculating the blade weight load, the mechanical torque load, andthe inertial load of the rotor blade 22, the in-plane blade root bendingmoment can be calculated using Equation (10) below:

InPlane Bending Moment=Blade Weight Load+Mechanical Torque Load+InertialLoad   Equation (10)

As mentioned, the processor 58 may also be configured to determinefurther blade loading components in addition to the out-of-plane bladeroot moment and the in-plane blade root moment, e.g. blade axial forces,out-of-plane shear forces, in-plane shear forces, and/or blade torsion.In certain embodiments, such load components may have lesser impact tothe overall blade root load state and may be omitted.

More specifically, in one embodiment, the processor 58 is configured tocalculate the blade axial force of each rotor blade 22 as a function ofat least one of rotor blade mass, wind speed, center of gravitylocation, gravity, and/or the rotor azimuth angle. Thus, in furtherembodiments, the processor 58 may also calculate the out-of-plane shearforce for each rotor blade 22 as a function of at least the rotorblade's axial load and the cone and tilt angles. In additionalembodiments, the processor 58 is configured to calculate the in-planeshear force of each rotor blade 22 as a function of at least one of therotor azimuth angle, rotor blade mass, blade radius, ApplicationPointIP,torque, filtered rotor acceleration, rotor inertia, gravity, aerodynamicdrag, wind speed, and/or center of gravity location. For example, in oneembodiment, the in-plane shear force for each rotor blade 22 can becalculated using Equation (11) below:

$\begin{matrix}{{{InPlane}\mspace{14mu} {Shear}\mspace{14mu} {Force}} = {{{\sin(\theta)}*{BladeMass}*{Gravity}} + \frac{{Drag}*{Wind}\mspace{14mu} {Speed}}{{ApplicationPointIP}*{BladeRadius}} - \frac{\frac{Torque}{Nblades}}{{ApplicationPointIP}*{BladeRadius}} + \frac{\frac{{RotorAccTc}*{RotorInertia}}{Nblades}}{CGloc}}} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

where

-   RotorAccTC is the first order low pass filter time constant for the    rotor acceleration signal and can be set based on the quality of the    speed measurement signal.

After calculating the individual blade load components, the processor 58is configured to calculate the overall blade root resultant moment ofthe rotor blade 22 based at least partially on the out-of-plane bladeroot bending moment and the in-plane blade root bending moment. Forexample, in one embodiment, the overall blade root resultant moment canbe calculated using Equation (12) below:

$\begin{matrix}{{{Blade}\mspace{14mu} {Root}\mspace{14mu} {Resultant}\mspace{14mu} {Moment}} = \sqrt{{{OutofPlane}\mspace{14mu} {Bending}\mspace{14mu} {Moment}^{2}} + {{InPlane}\mspace{14mu} {Bending}\mspace{14mu} {Moment}^{2}}}} & {{Equation}\mspace{14mu} (12)}\end{matrix}$

Referring now to FIGS. 7-10, various graphs illustrating the accuracy ofthe present disclosure as compared to measured data and/or previous loadestimation methods are illustrated. More specifically, as shown in FIG.7, a graph 100 of one embodiment of the estimated blade root resultantmoment 102 as calculated by the present disclosure and the measuredblade root resultant moment 104 as a function of time is illustrated. Asshown, the accuracy of the present disclosure is illustrated by theclose correlation between the estimated blade root resultant moment 102and the measured blade root resultant moment 104. In addition, as shownin graph 200 of FIG. 8, the data from FIG. 7 is plotted differently withthe measured blade root resultant moment on the x-axis and the estimatedblade root resultant moment on the y-axis. As shown, the slope of thedata 202 is approximately one to further illustrate the accuracy of theestimated blade root resultant moment of the present disclosure.

In contrast, as shown in the graph 300 of FIG. 9, the estimated thrust302 as calculated according to previous methods known in the art isplotted against the measured blade root resultant moment 304. As shown,the estimated thrust 302 is less correlated to the measured blade rootresultant moment 304 as the data provided in FIG. 7. In addition, asshown in graph 400 of FIG. 10, the data 402 is plotted with the measuredblade root resultant moment on the x-axis and the estimated thrust onthe y-axis. As shown, the data 402 is much less correlated than thegraph of FIG. 8, having a slope of much less than one.

Due to the accuracy of the algorithm for estimating the blade rootresultant moment of the present disclosure, the controller 26 of thepresent disclosure is capable of more accurately controlling loads ofthe wind turbine 10 without unduly sacrificing AEP. For example, asshown in FIG. 11, the processor 58 is configured to estimate the bladeroot resultant moment (e.g. MrB) and then determine the MrB Error usinga variety of suitable methods. For example, in one embodiment, the windturbine parameter estimator 56 is configured to calculate the MrB Errorbased on Equation (13) below:

MrB Error=max(MrB₁, MrB₂, MrB₃)−MrBLimit   Equation (13)

where

-   MrB₁ is the blade root resultant moment from a first rotor blade,-   MrB₂ is the blade root resultant moment from a second rotor blade,-   MrB₃ is the blade root resultant moment from a third rotor blade,    and-   MrBLimit is the blade root resultant moment limit that prevents    damage to the turbine components.

Based on the error, the controller 26 may determine and implement acontrol action needed for zero or near zero error. For example, thecontroller 26 may calculate a pitch angle for one or more of the rotorblades 22 and apply the new pitch constraint to reach zero error.Further, it should be understood that the control action as describedherein may encompass any suitable command or constraint by thecontroller 26. For example, in several embodiments, the control actionmay include temporarily de-rating or up-rating the wind turbine toprevent excessive loads on one or more of the wind turbine components.Up-rating the wind turbine, such as by up-rating torque, may temporarilyslow down the wind turbine and act as a brake to help reduce or preventloading. De-rating the wind turbine may include speed de-rating, torquede-rating or a combination of both. Further, as mentioned, the windturbine 10 may be de-rated by pitching one or more of the rotor blades22 about its pitch axis 28. More specifically, the controller 26 maygenerally control each pitch adjustment mechanism 32 in order to alterthe pitch angle of each rotor blade 22 between 0 degrees (i.e., a powerposition of the rotor blade 22) and 90 degrees (i.e., a featheredposition of the rotor blade 22). As such, in one embodiment, thecontroller 26 may command a new pitch setpoint (e.g. from 0 degrees to 5degrees), whereas in another embodiment, the controller 26 may specify anew pitch constraint (e.g. a constraint to ensure that subsequent pitchcommands are at least 5 degrees).

In still another embodiment, the wind turbine 10 may be temporarilyde-rated by modifying the torque demand on the generator 24. In general,the torque demand may be modified using any suitable method, process,structure and/or means known in the art. For instance, in oneembodiment, the torque demand on the generator 24 may be controlledusing the controller 26 by transmitting a suitable controlsignal/command to the generator 24 in order to modulate the magneticflux produced within the generator 24.

The wind turbine 10 may also be temporarily de-rated by yawing thenacelle 22 to change the angle of the nacelle 16 relative to thedirection of the wind. In further embodiments, the controller 26 may beconfigured to actuate one or more mechanical brake(s) in order to reducethe rotational speed of the rotor blades 22, thereby reducing componentloading. In still further embodiments, the controller 26 may beconfigured to perform any appropriate control action known in the art.Further, the controller 26 may implement a combination of two or morecontrol actions.

Referring now to FIG. 12, a flow diagram of method 500 according to oneembodiment of the present disclosure is illustrated. As shown, themethod 500 includes a first step 502 of measuring a plurality ofoperating parameters of the wind turbine. Another step 504 includesestimating an out-of-plane force acting on the rotor blade based atleast partially on the plurality of operating parameters. The method 500also includes a step 506 of determining an application point of theout-of-plane force along a span of the rotor blade. A further step 508includes estimating an out-of-plane moment of the rotor blade based atleast partially on the out-of-plane force and the application point.Another step 510 includes estimating an in-plane blade moment of therotor blade. The method 500 may then include a step 512 of calculatingthe load acting on the rotor blade based at least partially on theout-of-plane moment and the in-plane moment.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method for estimating a load acting on a rotorblade of a wind turbine, the method comprising: measuring, via one ormore sensors, a plurality of operating parameters of the wind turbine,estimating, via a processor, an out-of-plane and in-plane force actingon the rotor blade based at least partially on the plurality ofoperating parameters; determining, via the processor, an applicationpoint of the out-of-plane and in-plane force on a span of the rotorblade; estimating, via the processor, an out-of-plane moment of therotor blade based at least partially on the out-of-plane force and theout-of-plane application point; estimating, via the processor, anin-plane blade moment of the rotor blade based at least partially on thein-plane force and the in-plane application point; and, calculating, viathe processor, the load acting on the rotor blade based at leastpartially on the out-of-plane moment and the in-plane moment.
 2. Themethod of claim 1, wherein the load acting on the rotor bladecorresponds to a blade root resultant moment.
 3. The method of claim 1,wherein the plurality of operating parameters of the wind turbinecomprise at least one of the following: a thrust, a power, a speed, atorque, a pitch angle, a nodding moment, an overhang moment, a rotorazimuth angle, or a yawing moment.
 4. The method of claim 3, furthercomprising estimating the thrust utilizing at least one of thefollowing: a plurality of equations, one or more aerodynamic performancemaps, or one or more look-up tables.
 5. The method of claim 1, whereinestimating the out-of-plane force further comprises: determining anout-of-plane force distribution from a blade root to a blade tip of therotor blade; and integrating the out-of-plane force distribution toobtain an equivalent out-of-plane force.
 6. The method of claim 1,wherein determining the application point of the out-of-plane forcefurther comprises: determining a plurality of application points of aplurality of out-of-plane forces during operation of the wind turbinefor a plurality of wind speeds; and storing the plurality of applicationpoints in an aerodynamic performance map.
 7. The method of claim 1,wherein the application point of the out-of-plane force comprises aconstant value.
 8. The method of claim 3, further comprising estimatingthe in-plane moment of the rotor blade as a function of at least one ofa weight of the rotor blade, a mechanical torque load of the rotorblade, a speed of the rotor blade, a power of the rotor blade, or aninertial load of the rotor blade.
 9. The method of claim 8, furthercomprising calculating the gravity load of the rotor blade as a functionof at least one of the rotor azimuth angle, a mass of the rotor blade,gravity, a hub radius, or a center of gravity location of the rotorblade.
 10. The method of claim 9, further comprising calculating themechanical torque load of the rotor blade as a function of at least oneof a low-speed shaft torque, a blade radius, or the hub radius.
 11. Themethod of claim 10, further comprising calculating the inertial load ofthe rotor blade as a function of at least one of a rotor acceleration ora rotor inertia.
 12. The method of claim 11, further comprisingdetermining the rotor acceleration based at least partially on a rate ofchange of a rotor speed signal and filtering the speed signal via a lowpass filter.
 13. The method of claim 3, further comprising: estimatingan in-plane shear force of the rotor blade as a function of at least oneof the rotor azimuth angle, a rotor blade mass, a rotor blade radius,the application point, torque, filtered rotor acceleration, rotorinertia, gravity, aerodynamic drag, wind speed, or a center of gravitylocation; and calculating the load acting on the rotor blade based atleast partially on the in-plane shear force.
 14. A system formaintaining rotor blade loads of a wind turbine within predeterminedlimits while also maximizing power output, the system comprising: one ormore sensors configured to measure a plurality of operating parametersof the wind turbine; a controller configured with the one or moresensors, the controller comprising a processor configured to perform oneor more operations, the operations comprising: estimating anout-of-plane and in-plane force acting on the rotor blade based at leastpartially on the plurality of operating parameters; determining anapplication point of the out-of-plane and in-plane force on a span ofthe rotor blade; estimating an out-of-plane moment of the rotor bladebased at least partially on the out-of-plane force and the out-of-planeapplication point; estimating an in-plane blade moment of the rotorblade based at least partially on the in-plane force and the in-planeapplication point; and, calculating the load acting on the rotor bladebased at least partially on the out-of-plane moment and the in-planemoment.
 15. The system of claim 14, wherein the plurality of operatingparameters of the wind turbine comprise at least one of the following: athrust, a power, a speed, a torque, a pitch angle, a nodding moment, anoverhang moment, a rotor azimuth angle, or a yawing moment.
 16. Thesystem of claim 14, wherein estimating the out-of-plane root momentfurther comprises: determining an out-of-plane force distribution from ablade root to a blade tip of the rotor blade; and integrating theout-of-plane force distribution to obtain an equivalent out-of-planeforce.
 17. The system of claim 14, wherein determining the applicationpoint of the out-of-plane force further comprises: calculating aplurality of application points of a plurality of out-of-plane forcesduring operation of the wind turbine, wherein the plurality ofapplication points correspond to a plurality of wind speeds; and storingthe plurality of application points in an aerodynamic performance map.18. The system of claim 14, wherein the application point of theout-of-plane force comprises a constant value.
 19. The system of claim13, further comprising estimating the in-plane blade root moment of therotor blade as a function of at least one of a weight of the rotorblade, a mechanical torque load of the rotor blade, a speed of the rotorblade, a power of the rotor blade, or an inertial load of the rotorblade.
 20. A method for estimating an out-of-plane blade root moment ofthe rotor blade, the method comprising: measuring, via one or moresensors, a plurality of operating parameters of the wind turbine;estimating, via a processor, an out-of-plane force acting on the rotorblade based at least partially on the plurality of operating parameters;and determining an application point on the rotor blade of theout-of-plane force.